Huge FPS loss (~150 FPS) - CA SPR

[Flatlander]

Background:

Purchased a slightly use CA SPR which chrono'd 285fps when I got it. I was told it was a prom 100 spring so it is was only a touch low off the bat. I had to tinker with it and see what's inside and it's never been the same since...doh!

The gun is chrono'ing anywhere from 90 fps to 150fps. Occasionally I can get it to 230ish with the 100 spring. So there is some huge losses here that I cannot figure out for the life of me. The gun cycles, feeds and sounds great otherwise.

**Cylinder/nozzle compression tests were good. Plugging the inner barrel (exit end) and BB feed tube while blowing through the nozzle entrance of the hopup unit showed no leaks.

Parts:

**found a metal piston head and systema red piston originally so I swapped those out immediately.

- 7mm CA gearbox w/ stock gears
- Upgraded full cylinder (unknown brand)
- TM piston
- Unknown metal bearing spring guide (systema?)
- Upgraded brass tapered cylinder head (appears to be systema like the silent head)
- Looks like the stock plastic nozzle

What I did/tried:

- 3 springs: prom 100, stock TM, prom 90

- 3 rubbers: stock CA (?) that came with it, guarder clear and systema

- 2 hopup units: 2x stock CA and stock G&P

- TM piston head and Prometheus bearing POM heads

- Swapped upper receivers from my CA m4 with no change (m4 shoots great)

- Using the SPR upper on my m4 lower gives a 20fps boost (405fps) so it appears the hopup/inner barrel on the SPR is in good order.

- Hopup and inner barrel slide freely into the outter barrel and spring back

- Loosened full stock screw which attaches to the spring guide

- Reassembled the lower parts in different orders (ie. trigger pin, pistol grip, rear body pin)

Problem Areas Assumed to be Eliminated:

- Hopup unit, rubber and inner barrel

- Compression internals: good seals but only the piston head was swapped for comparison



So I am completely stumped on this one right now. My guess would be some sort of weird alignment issue but I haven't a clue. Any suggestions are much appreciated!

[dpvu]

It could be the nozzle and hopup unit do not align or seal properly. Maybe try your M4 mechbox in your SPR and see if you get a similar FPS loss. Since the SPR upper works great with your M4 lower, it seems like the problem is in the SPR mechbox or the interface between the mechbox and hopup.

[Corpo]

well, maybe you should try a second opinion... possibly broken chrono?

^possibly dumb idea

[Amos]

Sounds like the cylinder head isn't sealing very well.

Is your cylinder lubricated? Try dropping a couple drops of silicon oil in there...

I know with my VSR when it's running dry it fires 330 FPS.. but when lubed it fires a nice 465

[Flatlander]

@ dpvu: swapping mechboxes will be my very last resort. It took quite some time and tinkering to get my m4 to shoot the way it does and I'm scared to tear it down now, haha.

@ Corpo: Chrono seems fine as my other guns chrono where they should be. I also was firing my guns on sunday in a warehouse and couldn't even puncture the cardboard targets!

@ Amos: Are you sure you mean cylinder head? The oring is greased and plunging the piston into the cylinder manually revealed no leaks around the cylinder head. The cylinder head does appear to NOT be as snug in the cylinder as most I've seen; I'm wondering if the fast moving piston hits hard enough to break the seal and cause blow by through the cylinder head. I don't have a spare (quality) head to swap out unfortunately.

Also, how would you put a few drops of silicon oil into the cylinder head? I'm assuming you mean without disassembling the mechbox.

[BORDENSNIPER]

is ur tappet plate in good condition? no wear or cracks? might not be pushing the nozzle forward enough to seal. maybe its overgreased/undergreased.

[Flatlander]

Okay so I've found the problem: the full stock screw that screws into the spring guide. When it's tightened the fps is around 150. As I loosen it I can literally watch the fps rise the more I loosen it. When it is completely loose (as in putting no tension in the bolt) I get 290fps on a prom 90 spring...so about right. I did test this before but I think I didn't losen the bolt off enough.

I also noticed that when it's shooting properly I get a nice "thud" sound. When it's shooting low it's a much higher pitched snap. My guess is the higher pitched snap is from the air escaping between the hopup and nozzle at high velocities.

Now another problem: to get the gun to shoot right I need to have the bolt WAY too loose to the point where the stock wobbles. I think I may have to put a shim between the rear of the mechbox and the lower receiver so the bolt cannot pull the mechbox backwards. Any other ideas to fix this?

[Styrak]

You could also put washers/shims on where the bolt contacts the point inside the stock tube, to "shorten" the bolt.

[Flatlander]

@ BordenSniper: Tappet plate is in great shape

@ Stryak: I think I understand what you mean, but it appears any tension in the bolt causes it to go to shit.

Also noticed something else, when fully assembled and looking into the hopup feed tube from the magwell, you cannot see the nozzle at all. On my m4 I can see the nozzle slightly protruding. I thought maybe the nozzle was too short so I took it out and compared the size of it to a stock CA m4 nozzle using a caliper and they were pretty much identical (note: nozzle in the gun is actually an upgraded one with an o-ring).

So it's clearly a hopup-mechbox alignment or mating issue.

[Flatlander]

Okay so now I'm more confused than ever. Here's what I did (with the stock and stock screw completely off):

- Push the mag from the bottom, causing it to slighly rotate (picture the barrel being the axis of rotation) - get around 330fps consistently

- Pull the mag from the bottom, causing it to slightly rotate the opposite way now - get around 350fps consistently

- Let the mag hang freely and it's around 150-200 fps with the high pitched snap.

Any ideas why this is? Obviously some serious alignment issues.

[Dynamo]

did you have this issue with the orriginal nozzle?
also is your gearbox orriginal or upgraded?

i have been having feeding and powerloss issues with my M4 for a while.
i finally found out the problem after trying different hopups, barrels and pistons. it turns out that the new gearbox i got sits farther back than the orriginal one, that and my new nozzle was also a bit long by 0.3mm.
so my orriginal nozzle was too short and my new one was too long.
lucky for me that the new nozzle is metal and i was able to turn it down in length, so now it fits perfectly.

this caused really bad misfeeding, the BBs would drag the hopup sleeve lip into the barrel and jam. 4 pistons later i got it all fixed.

try sealing the end of your barrel and dryfire in single shot, see if you get back pressure in the cylender or if you piston still slams forward.

getting everything to line up is a real pain, especially when mix/matching parts.

it's also a bad idea to bolt your stock bolt through the back of your gearbox.
like the others have said, see how big the gap is between the back of the gearbox and the inside face of the lower reciver extension, and make your self a shim so the bolt wont pull the gearbox back.

[Flatlander]

@ Dynamo: It's the stock CA mechbox shell with some upgraded internals (see first post for list). I tried both nozzles and they both had troubles. I measured the lengths and diameters with a digital caliper and they were both withing ~.05mm of each other.

Now the funny thing is that I reassembled everything, including tightening the stock screw, and it was shooting 310 consistently on a promy 90 spring! The only thing I *think* I did differently was switch the 2 pistol grip screws in the other 2 holes. Who knows if that's what did it but I'm not touching it again until I have to upgrade it for outdoor limits...not looking forward to that! I might try to find another mechbox and throw it in there too.

EDIT: I also did try shimming the back of the mechbox. I had it such that the rear body pin was tougher to get in (took a little bit of slight hammering). Normally the rear body pin slides in with little to no resistance. The shimming had no affect and seemed to make it worse as even without the stock screw it wouldn't shoot right.

[mcguyver]

The order in which you assemble your gun can have an affect on it's performance. The very last screw you should tighten is the stock screw, even after the takedown pin and/or pivot pin are installed, and most certainly after the grip screws are in.

Sometimes it makes a difference, and sometimes not (tolerances and all), but as a matter of procedure, you have much fewer problems with doing the stock screw last.

I found that one out a long time ago.